***First i want to say I'm not sure if i should post this under Fullrange or Multiway. Since at this point, there is a high posibility of the need for a supertweeter i chose Multiway forums. I won't mind tho if moderators think it's more suited for Fullrange forums.

About 6 months ago, i started thinking about a DIY fieldcoil speaker. I know opinions vary quite a lot on this matter and so i will do my best to offer as much data as possible for things to be judged objectively. Since that time I have been talking with some fellow countrymen and audio enthusiasts about this project and these conversations helped me in making some of the design decisions. The work is still in progress and i hope your opinions and comments can help tune it to higher performance.

My personal taste in sound is that i like it when it comes out of large radiating surfaces. Large SD, large horn mouths etc. I think it's a necesity when one tries to reach real, live music levels and at the same time keep a good response to instruments' attack, sustain and decay. This is the reason i chose 12 inch diameter. I would've liked 15" better but it would've been too difficult to make it wide range. Perphaps a future project.

Field Coil Motor Structure:

I have spent a lot of time looking for other field coil projects, for any information on FC design or about available products and i have to say there isn't

much out there. From the pictures of opened FC units i could see a basic shape of the pole pieces. I have to say this is amongst the first design issues i encountered, there isn't much freedom to design these pole pieces, not like the case with permanent magnets anyway.
The next thing one can notice is that the new FCs are low impedance type and this is easily understandable. I have decided that my field coil will also work with 12-15V supply.
Underhung design means thick top plate. Of course this will cause a drop in flux density in the gap. The compromise here i found to be between flux density value [B] and voice coil excursion. Since this speaker will mostly be used horn loaded i sacrificied Xmax to make a thinner top plate. I use standard 1010 steel with a maximum of 0.13% carbon, the best i could get my hands on.
I ran simmulations in FEMM for different values of top plate thickness, FC's number of turns, dimension, wire diameter etc. Here are the results i stopped at:

I ran simmulations with 1010 steel as well as 416 steel (has slightly more carbon typical 0.15%) to see the differences.

1010 Steel [B] Curve over 20mm and 12mm:

416 Steel [B] Curve over 20mm and 12mm: (actually the curves are smoother had to lower resolution a bit because of my computer)

Magnetic gap:

AS it is, at 2.2 Amps we have 25W for the FC. I don't expect to have too much trouble with heat at this power. Test will tell tho, depending on how well the metal does the job of sinking the heat. The [B] value predicted by FEMM lies around 1.6 Tesla. In reality i believe this value will be smaller 1.4-1.5 T. In tests i will drive the FC with 3 amps and see how much it heats up.

In this design the gap width is 1.5mm. Somewhat large, I'm thinking to add a 0.5mm thick copper ring right in the gap (yellow part from picture below). Now
it's true i have never seen a picture with a FC unit having a faraday ring and I'm not sure if it's necesary. I guess the FC itself has a role in that too.

I'm also making a 1mm gap variant (without copper ring) in hopes to get more flux density. The FEMM simmulation shows:

Voice coil is 2" diameter. Haven't decided on the former material yet, and there is still some designing work to be done. I will use 0.13mm copper-clad aluminum wire (hoping to receive it Monday).
Data for the ECCA wire is as follows:

Cone will be made of japanese paper called washi (fullrange crowd might now why hehe). I got a few rolls of washi of different types and will test a few variants.

Next week work will start on cones, most probably will be made from 12 petals. I will test both straigth and curved cone. An exponential curved profile like in the picture below has a calculated flare rate of 0.387/cm which should correspond to a freq close to 1050Hz. Have to look into whizer cone as well.

Thats it for now, let me know what you think so far. I will post more info as i go with this project. I hope it will awake more interest into fieldcoils and
shed some light on the matter.

1.4 tesla is 14000 gauss so this seems like a very reasonable flux density in the gap, not too many PM speakers do better than this. I'm assuming (naively perhaps) that this design will have a pretty high BL?

__________________"To argue with a person who has renounced the use of reason is like administering medicine to the dead." - Thomas Paine

Yes, a high BL is mandatory for high efficiency. I am also looking to tilt the response upwards in frequency, as in higher sensitivity while going up in band. This will be very helpful when using horn loading.
Of course electromagnetic overdamping will be an option on how hot you run the FC.
But as you identified, i aim to get 1.4T and not overheat the FC.

I will post later about Voice coil and its part in the BL figure. A high length of wire makes a high BL but also increases mass and i want to keep mass as low as possible for efficiency and wide bandwidth. However i made some calculations on how much i gain either by increasing BL and mass alltogether or reducing BL and mass as well. I will post them later after some checking. BL on its own doesn't say the whole story there is also the issue of DC resistance of wire that i will touch on later as well.

Wondering about the baskets, are you going with stamped steel or will you use cast aluminum in the actual driver build? I would suspect that the cast aluminum are likely to be more rigid and less likely to exhibit any sort of resonances..

__________________"To argue with a person who has renounced the use of reason is like administering medicine to the dead." - Thomas Paine